Electrolysis cells, according to the prior art, have usually been made in one of two ways:
(a) The anode part and the cathode part both comprise the same material, and the anode part has an electrocatalytically active coating, or both parts comprise alloys having the same main components (see examined German Patent Application No. DE-AS 24 35 185, for instance); and PA1 (b) the anode and the cathode are located parallel to and at a distance from one another and are joined to one another via back plates made of two-layered metal strips (see German Patent Disclosure Document No. DE-OS 26 56 110).
Conventional joining methods for bipolar electrodes which are located parallel to each other and spaced from each other can be used.
Anode materials for bipolar electrodes usually use valve metals. They are used, conventionally, because they are dimensionally stable. The typical anode materials are titanium, tantalum, zirconium, niobium, tungsten. The foundation body of the anode material has an electrically conductive surface, for example a platinum metal, and platinum metal oxide, or a conductive metal oxide or oxide mixture resistant to the anolyte. Valve metals are metals which form non-conductive oxides which are resistant to the anolyte. The electrodes may be made in the form of expanded metal, net or grid. Expanded metal, net or grid anodes are preferred because of the larger electrocatalytically active surface and the desirable electrolyte flow which then can be attained.
The material for the cathode is selected from an electrically conductive substance which is resistant to the catholyte, usually steel, nickel, iron, or alloys of steel, nickel or iron. The cathode, like the anode, is preferably made of perforated material, and may be made from flat sheet or plates. The cathode, desirably, is coated on its surface with nickel or a nickel alloy or a nickel compound.
Joining non-compatible materials, such as tantalum for an anode and steel for a cathode, or titanium for an anode and steel for a cathode, causes difficulty. These metals cannot normally be welded to one another. In order to provide a connection between such non-compatible materials, an intermediate element was inserted therebetween made of a material which could be joined satisfactorily to both the anode material as well as the cathode material. A typical material used as a intermediate is copper. Copper, however, has a substantial disadvantage. Copper has no resistance to corrosion and, specifically, has no resistance to the electrolyte which is present in the environment in which the electrode is to operate.
It has been proposed to fabricate a bimetal element from two materials which normally cannot be welded together by roll-bonding or plate-bonding. An intermediate element used to connect the cathode and the anode which is made of roll-bonded bimetal cannot be used in electrodes, however, since the roll bond does not withstand the welding conditions of the respective metal of the intermediate element to the anode or cathode, respectively. The high temperatures required during welding, particularly for welding of tantalum for example, would destroy the bond and/or the intermediate bonding material.